Go another route if you only want it for Plex. They are a VERY expensive way to get a Plex server if all you are doing with it is Plex. You can add more RAM, swap drives, maybe do an SSD cache setup, or maybe add a PCI card of some kind. Synology units are not nearly as upgradeable as other options. If you want to cover that use-case of using 4k files knowing you will have situations where you do need to transcode, then yes HW acceleration becomes very important. Most of the time when 4k is being transcoded back to 4k, something is being done wrong. That almost always means a transcode of the 4k file down to 1080p or lower.

HW transcoding is used for converting 4k files when the client can't play the 4k file for whatever reason. As in, watch actual 4k with HDR on a 4k setup. HW transcoding is irrelevant if you want to 'stream 4k'.

I'm just zeroing in on this comment right here to provide some understanding you might not already have. If I'm just playing content on my main TV, it Direct Plays and there is no fuss.What would be the best NAS for Plex that has say 5 or 6 bays with hw transcoding to watch one or 2 streams of 4k? Works like a charm and I can transcode about 3 simultaneous remote 720p streams before things start getting hot. I do this with a 2012 i5 Mac Mini and a QNAP 870 mounted via AFP. The only thing I would strongly recommend is to make sure both parts of the split system are wired (ethernet). And then you have that massive NAS (any size, any specs) you want to store everything and you mount it via SMB/AFP/NFS shares to the NUC/Mini. Your other clients (Roku/Web/Chromecast/FireTV/Phone) will use the NUC/Mini to transcode but there is no issue there because they have ample CPU to do so. Also, with PHT there is no need to transcode-it will Direct Play anything with minimal resource usage. The good thing about using a split system like that is you also get a Plex Home Theater client out of it (via connecting the Mini or NUC to your TV via HDMI your main TV).

Something like an Intel NUC or Mac Mini to run PMS/PHT on, and the NAS to store the media. Some of the newer QNAP's can handle this but they are extremely expensive ($2500+ w/ drives).

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